Group III nitride compound semiconductor are direct-transition semiconductors exhibiting a wide range of emission spectra from UV to red light when used in an element such as a light-emitting device, and have been used in light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs). In addition, due to their broad band gaps, divices employing the aforementioned semiconductors are expected to exhibit reliable operational characteristics at high temperature as compared with those employing semiconductors of other types, and thus application thereof to transistors such as FETs has been energetically studied. Moreover, since Group III nitride compound semiconductors contain no arsenic (As) as a predominant element, application of Group III nitride compound semiconductors to various semiconductor devices has been longed for from the environmental aspect. Generally, these Group III nitride compound semiconductors are formed on a sapphire substrate.
However, when a Group III nitride compound semiconductor is formed on a sapphire substrate, misfit-induced dislocations occur due to difference between the lattice constant of sapphire and that of the semiconductor, resulting in poor device characteristics. Misfit-induced dislocations are threading dislocations which penetrate semiconductor layers in a longitudinal direction (i.e., in a direction vertical to the surface of the substrate), and Group III nitride compound semiconductors are accompanied by the problem that dislocations in amounts of approximately 109 cm−2 propagate therethrough. The aforementioned dislocations propagate through layers formed from Group III nitride compound semiconductors of different compositions, until they reach the uppermost layer. When such a semiconductor is incorporated in, for example, a light-emitting device, the device poses problems of unsatisfactory device characteristics in terms of threshold current of an LD, service life of an LED or LD, etc. On the other hand, when a Group III nitride compound semiconductor is incorporated in any of other types of semiconductor devices, because electrons are scattered due to defects in the Group III nitride compound semiconductor, the semiconductor device comes to have low mobility. These problems are not solved even when another type of substrate is employed.
The aforementioned dislocations will next be described with reference to a schematic representation shown in FIG. 6. FIG. 6 shows a substrate 91, a buffer layer 92 formed thereon, and a Group III nitride compound semiconductor layer 93 further formed thereon. Conventionally, the substrate 91 is formed of sapphire or a similar substance and the buffer layer 92 is formed of aluminum nitride (AlN) or a similar substance. The buffer layer 92 formed of aluminum nitride (AlN) is provided so as to relax misfit between the sapphire substrate 91 and the Group III nitride compound semiconductor layer 93. However, generation of dislocations is not reduced to zero. Threading dislocations 901 propagate upward (in a vertical direction with respect to the substrate surface) from dislocation initiating points 900, penetrating the buffer layer 92 and the Group III nitride compound semiconductor layer 93. When a semiconductor device is fabricated by depositing various types of Group III nitride compound semiconductors of interest on the Group III nitride compound semiconductor layer 93, threading dislocations further propagate upward, through the semiconductor element, from dislocation arrival points 902 on the surface of the Group III nitride compound semiconductor layer 93. Thus, according to conventional techniques, problematic propagation of dislocations cannot be prevented during formation of Group III nitride compound semiconductor layers.
In recent years, in order to prevent propagation of the threading dislocations, techniques employing lateral growth of crystal have been developed. According to the techniques, a mask partially provided with an array of slits, which is formed from a material such as silicon oxide or tungsten, is provided on a sapphire substrate or a Group III nitride compound semiconductor layer, and crystal growth is elicited to proceed laterally on the mask, with the slits serving as a seed. Threading dislocations, however, propagate upside on the upper portion of the window part. In order to prevent threading dislocations from propagating on the window part, the upper portion of the mask should be covered through lateral growth, and further a second mask should be formed in stripe pattern on the upper portion of the window on which the mask is not formed and then the lateral growth is executed on the mask again. In short, three times of forming a Group III nitride compound semiconductor process and two times of mask forming process, each of which is a completely different process, needed to be carried out.